Magnetic materials can be divided into two major classes: permanent magnetic materials (also referred to as hard magnetic materials) and temporary magnetic materials (also referred to as soft magnetic materials).
The permanent magnets are characterized by a large remanence, so that after removal of a magnetizing force, a high flux density remains. The permanent magnets tend toward large hysteresis loops, which are the closed curves showing the variation of the magnetic induction of a magnetic material with the external magnetic field producing it, when this field is changed through a complete cycle. Permanent magnets are commonly physically hard substances and are, therefore, called hard magnets.
The temporary or soft magnets have low values of remanence and small hysteresis loops. They are commonly physically softer than the hard magnets and are known as soft magnets. Ideally, the soft magnets should have large values of permeability (μ) up to a high saturated flux density. The value of the permeability (μ) is the ratio B/H, where H represents the applied magnetic field, or magnetic force, expressed in amperes per meter (A/M) and B is the magnetic flux density induced in the material, and it is expressed in teslas (one tesla being equal to one weber per meter square (W/m2)).
Soft magnetic materials are usually for applications where they have to canalize a varying magnetic flux. They are conventionally used for manufacturing transformers, inductance for electronic circuits, magnetic screens, stator and rotor of motors, generators, alternators, field concentrators, synchroresolver, etc. A soft magnetic material has to rapidly react to the small variations of an external inducing magnetic field, and that, without heating and without affecting the frequency of the external field.
Therefore, soft magnets are usually used with alternating currents, and for maximum efficiency, it is essential to minimize the energy losses associated with the changing electric field. The energy losses, or core losses, as they are sometimes called, result in conversion of electric energy to thermal energy. The losses are usually expressed in terms of watts/kg (W/kg) for a given flux density (in teslas) at a given frequency (in Hertz). There are two principal mechanisms by which energy or core losses occur. These are hysteresis losses and eddy current losses. Soft magnetic materials have to have a small hysteresis loop (a small coercive field HC) and a high flux density (B) at saturation.
As well explained in U.S. Pat. No. 6,548,012, hysteresis losses are due to the energy dissipated by the wall domain movement and they are proportional to the frequency. They are influenced by the chemical composition and the structure of the material.
Eddy currents are induced when a magnetic field is exposed to an alternating magnetic field. These currents which travel normal to the direction of the magnetic flux lead to an energy loss through Joule (resistance) heating. Eddy current losses are expected to vary with the square of the frequency, and inversely with the resistivity. The relative importance of the eddy current losses thus depends on the electrical resistivity of the material.
In prior art, soft magnetic parts for alternative current of low and medium frequency applications (between 50 Hz and 50 000 Hz) have been produced using basically two different technologies, each having their advantages and limitations.
The first and widely used, since the end of the 19th century, consists of punching and stacking steel laminations. This well-known process involves material loss since scrap material is generated from notches and edges of the laminations when stamping. This material loss could be very costly with some specific alloys. This process also requires a default free roll of material of dimensions greater than the dimensions of the part to be produced. The laminations have the final geometry or a subdivision of the final geometry of the parts and can be coated with an organic and/or inorganic insulating material. Every imperfection on the laminations like edges burr decreases the stacking factor of the final part and thus its maximum induction. Also, mass production of laminations prevents design with rounded edges to help copper wire winding. Due to the planar nature of the laminations, their use limits the design of devices with 2 dimensions distribution of the magnetic field. Indeed, the field is limited to travel only in the plane of the laminations.
The cost of the laminations is related to their thickness. To limit energy losses generated by eddy currents, as the magnetic field frequency of the application increases, laminations thickness must be decreased. This increases the rolling cost of the material and decreases the stacking factor of the final part due to imperfect surface finish of the laminations and burrs and the relative importance of the insulating coating. Laminations are thus well suited but limited to low frequency applications.
The second process for the production of soft magnetic parts for AC applications, well-known since the beginning of the 20th century, is a variant of the mass production powder metallurgy process where particles used are electrically isolated from each other by a coating (U.S. Pat. Nos. 421,067; 1,669,649; 1,789,477; 1,850,181; 1,859,067; 1,878,589; 2,330,590; 2,783,208; 4,543,208; 5,063,011; 5,211,896). To prevent the formation of electrical contacts between the powder particles, and thus to reduce the eddy current losses, the powder particles are not sintered for AC applications. Parts issued from this process are commonly named “soft magnetic composites or SMC”. Obviously, this process has the advantage of eliminating material loss.
SMC are isotropic and thus offer the possibility of designing components which allow the magnetic fields to move in the three dimensions. SMC allow also the production of rounded edges with conventional powder metallurgy pressing techniques. As mentioned above, those rounded edges help winding the electric conductors. Due to the higher curvature radius of the rounded edges, the electrical conductors require less insulation. Furthermore, a reduction in the length of the conductors due to the rounded edges of the soft magnetic part is a great advantage, since it allows the amount of copper used to be minimized as well as the copper loss (loss due to the electrical resistivity of the electrical conductor carrying the current in the electromagnetic device).
With rounded edges, the overall dimension of the electrical component could be reduced, since electrical winding could be partially inlaid within the volume normally occupied by the soft magnetic part. In addition, due to the isotropy of the material and the gain of freedom of the pressing process, new designs that increase total yield, decrease the volume or the weight for the same power output of electric machines are possible, since a better distribution or movement of the magnetic field in the three dimensions is possible.
Another advantage of the powder metallurgy process is the elimination of the clamping mean needed to secure laminations together in the final part. With laminations, clamping is sometimes replaced by a welding of the edges of laminations. Using the later approach, the eddy currents are considerably increased, and the total yield of the device or its frequency range application is decreased.
The limitation of the SMC is their high hysteresis losses and low permeability compared to steel laminations. Since particles must be insulated from each other to limit eddy currents induction, there is a distributed air gap in the material that decreases significantly the magnetic permeability and increases the coercive field. Additionally, to prevent the destruction of the insulation or coating, SMC can very hardly be fully annealed or achieve a complete recrystallisation with grain coarsening. The temperatures reported for annealing SMC without loosing insulation are about 600° C. in a non-reducing atmosphere and with the use of partially or totally inorganic coating (U.S. Pat. Nos. 2,230,228; 4,601,765; 4,602,957; 5,595,609; 5,754,936; 6,251,514; 6,331,270 B1; PCT/SE96/00397). Although the annealing temperature commonly used is not sufficient to completely remove residual strain in the particles or to cause recrystallisation or grain growth, a substantial amelioration of the hysteresis losses is observed.
Ultimately, for all the soft magnetic composites with irregular or spherical particles developed for AC applications until now, even if residual strain would have been removed and grain growth would have been possible at temperatures used for the annealing cycle of finished parts, metallic grain dimension is limited to the size of the particles. This small grain size limits the possibility of increasing the permeability, decreasing the coercive field or simply, the hysteresis losses in the material. Indeed, the smaller the metallic grains are, the higher is the number of grain boundaries, and more energy is required for moving the magnetic domain walls and increasing the induction of the material in one direction. Therefore, the resulting total energy losses (or core losses) of SMC parts at low frequency (below 400 Hz) is greater than the total energy losses obtained with laminations. The low permeability values require also more copper wire to achieve the same induction or torque in the electromagnetic device. An optimized three dimensions and rounded winding edges design of the part made with the SMC with irregular or spherical particles can partially or completely compensate those higher hysteresis losses and low permeability values encountered with SMC material at low frequency.
Some attempts have been made to develop more performing inorganic coatings and processes for conventional soft magnetic composites that would allow a full annealing of compacts and even recrystallisation without losing too much electrical insulation between particles (U.S. Pat. Nos. 2,937,964; 5,352,522; EP 0 088 992 A2; WO 02/058865). These prior art documents teach a heat treatment at around 1000° C. or less to consolidate particles by the diffusion or interaction of the insulating material of each particle. In all these cases, the goal is to produce a soft magnetic composite with discontinuous, separated soft magnetic particles joined by a continuous electrical insulating medium. The DC magnetic properties (coercive field and maximum permeability) of the produced composite are far inferior to those of the main wrought soft magnetic constituting material in the form of lamination, and thus, hysteresis losses in an AC magnetic field are higher and the electrical current or the number of turns of copper wire required to reach the same torque must be higher. Properties of those composites are well suited for applications frequency above 10 KHz to 1 MHz. If power frequencies are targeted (US Patents EP 0 088 992 A2 and WO 02/058865), the design of the component must compensate for the lower permeability and higher hysteresis losses of the material.
Finally, some people who have discovered the benefit of using lamellar particles for doing soft magnetic components have developed coating able to sustain annealing temperature, that is to say temperatures which are high enough to remove the major part of the remaining strain in the parts (U.S. Pat. Nos. 3,255,052; 3,848,331; 4,158,580; 4,158,582; 4,265,681). Once again, magnetic properties and energetic losses in an AC magnetic field at frequencies under 400 Hz are not those reached with good lamination steel or silicon steel used commercially, since metallic diffusion between soft magnetic particles is avoided to keep high electrical resistivity in the composite.
Since all the actual soft magnetic composite are discontinuous metallic media, the mechanical strength of the material is limited to the strength of the insulating coating. When the material breaks, it is de-cohesion that occurs between metallic particles, in the organic or inorganic (vitrous/ceramic) coating. The mechanical behavior of the SMC is thus fragile with no possibility of plastic deformation and the strength is always far lower than that of metallurgically bonded materials. It is an important limitation of the SMC.
Also known in the prior art are the sintered iron non coated powder components currently used to make parts for DC magnetic applications. These sintered parts have low resistivity and are generally not used in AC applications. In the literature or patents, when sintering treatments (metal to metal) or metallic diffusion are involved, soft magnetic parts produced are for DC applications where eddy currents are not a concern (U.S. Pat. Nos. 4,158,581; 5,594,186; 5,925,836; 6,117,205 for example) or for non-magnetic applications like structural parts.